It is said that the sea covers about 70% of the earth surface, and has a thermal capacity about 1000 times as much as that of the atmosphere. Accordingly, large variations in temperature of seawater greatly affect the state of the atmosphere and largely change the climate and weather all over the world (e.g., El Nino etc.). To address this, various effects on the earth can be predicted and prevented by collecting oceanographic internal information including the temperature of seawater and grasping variation in oceanographic information. Several proposals have already been made as systems for collecting such oceanographic information (e.g., see Patent Literatures 1 and 2).
Patent Literature 1 discloses a system that collects oceanographic information using a movable unit that periodically sinking and floating upward repeatedly. This system is configured such that prescribed observation is performed during upward floating of the movable unit, and observation data is transmitted to the outside when the unit reaches the surface of the sea. The movable unit includes an adjustable ballast receiver. The volume of the adjustable ballast receiver is increased or reduced by injecting and draining ballast oil into and from the adjustable ballast receiver. The specific gravity of the movable unit is adjusted by changing in volume of the movable unit in the sea. Thus, change in volume of the adjustable ballast receiver adjusts the specific gravity of the movable unit, thereby allowing the movable unit to float upward and sink at a desired speed. An oceanographic information collection system disclosed in Patent Literature 1 can be referred to as a drift observation type, because the movable unit is not moored to the sea bottom.
Patent Literature 2 discloses a system in which a long-term observation station for collecting oceanographic information is arranged at a deep sea depth, and a data transmission buoy that periodically floats upward and sinks in a repeated manner transmits observation data to a ground base, thereby collecting oceanographic information. In this system, the data transmission buoy has a configuration capable of floating and sinking by means of a winch driving device installed at a relay base or a long-term observation station. In the oceanographic information collection system disclosed in Patent Literature 2, the long-term observation station is moored to the sea bottom. Accordingly, this system can be referred to as a fixed-point observation system.